docs: def PartialDynamicSystem(self, ieq, variable):
          """
          returns dynamical system blocks associated to output variable
          """
          if ieq == 0:
              # C1=-f*Tmax*sign(w1-w2)
              if variable == self.variables[0]:
                  ut = Variable('unsigned clutch friction torque', hidden=True)
                  b1 = Gain(self.commands[0], ut, self.Tmax)
                  dw = Variable('Delta rotationnal speed', hidden=True)
                  sdw = Variable('Sign of delta rotationnal speed')
                  b2 = WeightedSum(
                      [self.physical_nodes[0].variable, self.physical_nodes[1].variable], dw, [-1, 1])
                  b3 = Sign(dw, sdw)
                  b4 = Product(ut, sdw, variable)
                  return[b1, b2, b3, b4]
          elif ieq == 1:
              # C1=-C2
              if variable == self.variables[0]:
                  return [Gain(self.variables[1], variable, -1)]
              if variable == self.variables[1]:
                  return [Gain(self.variables[0], variable, -1)]